The present invention relates to methods and apparatuses for manufacturing a framework for a shadow mask used in cathode ray tubes. In particular, the present invention relates to methods and apparatuses for forming end faces on which a shadow mask is mounted, in which when ends of two opposing side frames positioned in parallel of a rectangular framework are formed into, for example, a curved shape, and a shadow mask is stretched on the end faces, a cylindrical surface formed by the shadow mask can have cylindricity and curvature with high precision.
In general, a shadow mask is mounted in a cathode ray tube by stretching it on a framework, which is assembled into a rectangular shape by means of welding etc. Usually, the framework is constructed by fixing a pair of long-side frames arranged horizontally to upper and lower ends of a pair of short-side frames, which are arranged vertically at a distance. Then, a shadow mask is stretched on screen side faces of the long-side frames.
The end faces of the long-side frames on which a shadow mask is stretched are usually formed into a predetermined curved shape (e.g. a circular arc shape with a radius R) so that their center portions become convex to the screen side.
Conventionally, the formation of curved end faces has been carried out by cutting or grinding after assembling the short-side and long-side frames into a rectangular form.
In the above-mentioned method, it is necessary to fix firmly the rectangular framework to be worked on. However, when the framework is fixed firmly, a deformation is generated in the rectangular framework. Furthermore, a rectangular framework bonded by welding often has a little distortion, and when it is fixed firmly in a working apparatus under this condition, a deformation resulting from the distortion is generated. Any of the above-mentioned deformations will be released when the framework is released from fixation after being worked on. Thus, no matter how precisely curved end faces are formed on a working apparatus, when the framework is released from fixation after being worked on, so-called return distortion is generated in a worked portion. Due to the return distortion, the precision of the cylindricity and curvature of a shadow mask surface is difficult to be increased. Furthermore, in order to prevent return distortion, there is a method of placing the rectangular framework precisely by a three-point supporting method. However, a high-level skill is required for the fixing method, and also productivity is inferior.
In the cutting or grinding process, when the strength of fixing the rectangular framework to a working apparatus is decreased so as to prevent the above-mentioned generation of return distortion, it is required to carry out the working under a light load, so that it takes a long working time.
Moreover, labor is required for setting and removing the rectangular framework to and from a working apparatus, and it is extremely inefficient.
It is an object of the present invention to provide methods and apparatuses for manufacturing a framework for a shadow mask that solve the above-mentioned conventional problems, in which precision is increased for forming end faces of a rectangular framework on which a shadow mask is mounted, working time is shortened considerably, operations of setting and removing the rectangular framework to and from a working apparatus are simplified, and working precision and production efficiency are satisfactory.
In order to accomplish the above object, the present invention provides methods and apparatuses as follows:
A first method for manufacturing a framework for a shadow mask according to the present invention includes cutting ends on a screen side of frames constituting two opposing sides of a framework in a predetermined shape by shearing, thereby obtaining end faces on which a shadow mask is stretched, wherein the framework is assembled in an approximately rectangular form.
A second method for manufacturing a framework for a shadow mask according to the present invention includes: assembling a framework in an approximately rectangular form; grinding a bottom face of the framework, thereby forming a basis plane; and cutting ends on a side opposite to the basis plane of frames constituting two opposing sides of the framework in a predetermined shape by shearing, thereby obtaining end faces on which a shadow mask is stretched.
According to the above first and second methods, the end faces on which a shadow mask is stretched are formed by shearing, and the shearing is carried out in only about several seconds. Thus, working time can be shortened considerably, and productivity of a framework for a shadow mask is improved significantly.
In the above second method, it is preferable that, when grinding the bottom face of the framework, the framework is placed on a grinding surface that is running, and the bottom face of the framework is ground while restricting movement of the framework in the running direction of the grinding surface. According to such a construction, in the subsequent shearing process, a basis plane for press-fixing the framework to an apparatus can be obtained at the bottom face speedily.
In the above, it is preferable that the bottom face of the framework is ground while substantially no pressurizing force other than the self-weight of the framework is applied in the direction perpendicular to the grinding surface. Accordingly, no external force other than the self-weight acts on the framework when forming the basis plane. Thus, the so-called return distortion is not generated after the grinding, and a basis plane having a very high flatness can be obtained.
In the above second method, it is preferable that when the shearing is carried out, the framework is fixed using the basis plane as an application surface. Accordingly, the so-called return distortion is not generated after the shearing. Furthermore, precision of the distance between the basis plane and the end faces formed by shearing can be increased, and also the generation of a tilt of the end faces with respect to the basis plane can be prevented.
In the above second method, it is preferable that a basis point for positioning is provided at one point in the framework, and that when ends of the frames constituting two opposing sides are sheared respectively, the positioning of each of the frames in a longitudinal direction is carried out using the basis point. Accordingly, the longitudinal positions of the two end faces formed on the two opposing sides can be allowed to coincide, so that a cylindrical surface formed by a shadow mask stretched on the end faces that are formed into, for example, a curved shape can realize very high cylindricity and accurate curvature.
A grinding apparatus for a framework for a shadow mask according to the present invention includes: a grinding surface that is running; and a stopper for preventing a framework from moving in a running direction of the grinding surface when the framework is placed on the grinding surface, wherein the framework is assembled in an approximately rectangular form. According to this apparatus, a basis plane for press-fixing the framework to an apparatus in the subsequent shearing process can be obtained speedily.
In the grinding apparatus, it is preferable that substantially no pressurizing force other than the self-weight of the framework is applied in the direction perpendicular to the grinding surface. Accordingly, when forming the basis plane, no external force other than the self-weight acts on the framework. Thus, the so-called return distortion is not generated after the grinding, and a basis plane with a very high flatness can be obtained.
Furthermore, in the grinding apparatus, it is preferable that the grinding surface is a circulating grinding belt, with the grinding belt sliding on a flat-plate shaped bed, and the framework is placed so as to face the bed. Accordingly, a basis plane with a high flatness can be obtained with a simple structure.
A shearing apparatus for a framework for a shadow mask according to the present invention includes: a die for cutting faces of frames constituting two opposing sides of a framework in a predetermined shape by shearing, wherein the framework is assembled in an approximately rectangular form and has a bottom face as a basis plane, and the faces are on a side opposite to the basis plane; a frame position controlling unit for positioning each of the frames in a longitudinal direction; a base on which the basis plane is applied; and a clamper for press-fixing the framework on the base. According to this apparatus, the end faces on which a shadow mask is stretched can be formed by shearing, so that working time can be shortened considerably, and productivity is improved significantly. Furthermore, when shearing is carried out, because the framework is press-fixed on the base at the basis plane, the framework is not deformed when it is press-fixed. Accordingly, the so-called return distortion is not generated when the framework is released from the press-fixing after the shearing, and end faces on which a shadow mask is mounted can be formed with high precision.
In the shearing apparatus, it is preferable that when the frames constituting two opposing sides are sheared respectively, positioning of each of the frames is carried out by the frame position controlling unit using a common basis point provided in the framework. Accordingly, longitudinal positions of the two end faces formed on the two opposing sides can coincide, so that a cylindrical surface formed by a shadow mask stretched on the end faces that are formed into, for example, a curved shape can realize very high cylindricity and accurate curvature.